Common Ion Effect

If in an aqueous solution of a weak electrolyte, a strong electrolyte is added having an ion common with the weak electrolyte, then the dissociation of the weak electrolyte is decreased or suppressed. The effect by which the dissociation of weak electrolyte is suppressed is known as common ion effect.

For example: In an aqueous solution of CH₃COOH, a small amount of CH₃COONa (strong electrolyte) has been added.

Due to the presence of common CH₃COO^– (aq) ions, the equilibrium will be shifted to the left.

Thus, the dissociation of CH₃COOH will get decreased or suppressed.

Hydrolysis of Salts and the pH of their Solutions

Salt Hydrolysis

Salt + water ⇌ Acid + Base

Hydrolysis is a process which is reverse of neutralization reaction.

Hydrolysis of Salts of Strong Acids and Weak Base

e.g., NH₄Cl, NH₄NO₃, etc.

After hydrolysis solution will be acidic pH < 7

Since only cations of the salt participate in the hydrolysis, it is known as cationic hydrolysis.

Hydrolysis of Salts of Strong Base and Weak Acids

Salts in this category are CH₃COONa, Na₂CO₃, Na₃PO₄, etc.

After hydrolysis solution will be basic pH > 7

In this type of hydrolysis, only anions of salt take part in the hydrolysis, it is known as anionic hydrolysis.

Hydrolysis of Salts of Weak Base and Weak Acids

Salts belong to this type are: CH₃COONH₄, (NH₄)₂CO₃ etc.

pH of solution depends upon the relative strengths of acid and base.